Although smallpox was eradicated in 1977, the possibility of a bioterrorist attack with the deadly virus has health officials worried. But a new study reports that should the virus re-emerge, those who had previously been vaccinated would largely be immune to the disease.

Researchers analyzed 50 years of blood chemistry data from 246 participants in the ongoing Baltimore Longitudinal Study on Aging and looked at various samples for the same individual over time to check his or her immune response.